Organic materials such as thermoplastic resins, thermosetting resins, natural rubbers, synthetic rubbers, mineral oils, lubricants, adhesives and paints are deteriorated, together with depolymerization and crosslinking, by the action of heat, oxygen, etc. during their production, processing or use. Due to the deterioration, their merchandize values are markedly lowered. In order to prevent the deterioration, i.e. thermal degradation, thermal oxidative degradation or the like, various phenol type antioxidants, phosphorus type antioxidants or the like have been added to organic materials. Among them, phosphorus type antioxidants such as phosphite compounds or phosphonite compounds have been widely used as processing stabilizers which cope with high temperature and high shear in processing steps of synthetic resins or synthetic rubber, because they are superior in retaining melt flow rates of synthetic resins or synthetic rubber and in preventing their heat-discoloration.
Typical examples of the phosphorus type antioxidants include Distearyl pentaerythritol diphosphite, Tris(2,4-di-t-butylphenyl)phosphite, Bis(2,4-di-t-butylphenyl) pentaerythritol diphosphite, Tetrakis(2,4-di-t-butylphenyl) 4,4'-biphenylene diphosphite and Bis(2,6-di-t-butyl-4-methylphenyl) pentaerythritol diphosphite
However, these usual phosphorus type antioxidants are not, satisfactory enough in stabilizing effects for preventing thermal degradation and thermal oxidative degradation and more improved antioxidants have been desired. For example, though Bis(2,4-di-t-butylphenyl) pentaerythritol diphosphite imparts excellent processing stability at high temperature, it is easily hydrolyzed and loses its processing stability effect. As a result of its hydrolysis during storage, it cannot stabilize organic materials any more. It also has a problem that it erodes processing apparatuses because of phosphorous acid, etc. produced by its hydrolysis. Tris(2,4 -di-t-butylphenyl) phosphite, though excellent in hydrolysis resistance, its processing stabilizing effect is not satisfactory enough.
Particularly in the fields of synthetic resins, since processing temperature has been getting higher recently in order to improve their productivity, processing stabilizers excellent in resistance to hydrolysis and imparting high processing stability have processing become desired more earnestly.